1. Field of the Invention
The present invention relates to a structure of a semiconductor laser diode and a method for producing the laser diode.
2. Related Prior Art
Japanese Patent published as A-H11-112098 has disclosed a distributed feedback (DFB) laser formed on the InP substrate. On the InP substrate with a feedback grating is grown in sequence the guiding layer, the spacer layer, and the MQW layer. The temperature profile to grow these layers shows a steep rise within 2 minutes and without any overshooting to the condition under which each layer is grown and makes the temperature drift stable within ±5° C. Moreover, the InGaAsP guiding layer starts its growth before the mass transportation of indium atoms occurs. Thus, the method disclosed in this patent prevents the decomposed InAsP layer from piling in the valley portion of the grating when the double hetero structure for the DFB-LD is formed on the InP substrate with the grating by the Organic Metal Vapor Phase Epitaxy (OMVPE) method.
The background of the present invention is that, in the DFB-LD, to keep the shape of the feedback grating is quite important for the single mode emission. When the InP substrate with the undulation for the feedback grating on the surface thereof is raised in the temperature as supplying the gas sources for the group V element, such as arsine AsH3 and phosphine PH3, an InAsP decomposed layer may be piled in the valley portion of the undulation, which degrades the shape of the feedback grating. The refractive index necessary to diffract light may be unable to obtain, consequently, the single mode oscillation can not be secured. Therefore, various methods has been proposed to maintain the dimensional quality of the feedback grating, in particular, the flow rate of the gas source for the group V element may be adjusted during the raising of the growth temperature.
When a semiconductor layer is grown immediately on the InP substrate with the grating on the surface thereof by the OMVPE method, the layer ordinarily called as the guide layer, first, the InP substrate is raised in the temperature thereof under an atmosphere including the group V element. The practical growth of the guide layer does not start until the temperature of the InP substrate becomes satisfactorily stable. This is because an overshoot of the temperature, i.e., the temperature of the substrate exceeds the growth temperature, occasionally appears in a range from several decades to around a hundred centigrade. Therefore, it is necessary to set a waiting time of about 10 minutes for the temperature being stable enough, and by adjusting the flow rate of the gas source of the group V element during this waiting, the dimensional quality of the grating may be secured.
The semiconductor material system for the DFB-LD of the present invention includes a separated confinement hetero-structure (SCH) layer made of AlGaInAs, which is quite different from the conventional system made of InGaAsP/InP. When this SCH layer is grown on the substrate with the grating, an intermediate layer of the InAsP is formed to secure the dimensional shape of the grating during the raising of the temperature as supplying the phosphine PH3 containing a minute amount of the arsine AsH3. The grating is comprised of the InP substrate, the InAsP intermediate layer, and the AlGaInAs SCH layer. However, the series resistance of this DFB-LD along the stacking direction of semiconductor layers does not show an anticipated resistance. This may be due to the band structure between the InAsP and the AlGaInAs. Therefore, an object for the DFB-LD including AlGaInAs material is an innovative band structure to overcome the subject between the InAsP and the AlGaInAs.
The present invention, performed to solve the above subject, is to provide a semiconductor laser diode with a feedback grating comprised of the AlGaInAs and the InP without the InAsP, and to provide a method for manufacturing such laser diode with a new structure.